1. Field of the Invention
The disclosed invention relates to RAID array controllers, and more particularly to a method and computer program product for storing configuration information with historical data on disk.
2. Background Art
There are many applications, particularly in a business environment, where there are needs beyond what can be fulfilled by a single hard disk, regardless of its size, performance or quality level. Many businesses can't afford to have their systems go down for even an hour in the event of a disk failure. They need large storage subsystems with capacities in the terabytes. And they want to be able to insulate themselves from hardware failures to any extent possible. Some people working with multimedia files need fast data transfer exceeding what current drives can deliver, without spending a fortune on specialty drives. These situations require that the traditional “one hard disk per system” model be set aside and a new system employed. This technique is called Redundant Arrays of Inexpensive Disks or RAID. (“Inexpensive” is sometimes replaced with “Independent”, but the former term is the one that was used when the term “RAID” was first coined by the researchers at the University of California at Berkeley, who first investigated the use of multiple-drive arrays in 1987. See D. Patterson, G. Gibson, and R. Katz. “A Case for Redundant Array of Inexpensive Disks (RAID)”, Proceedings of ACM SIGMOD '88, pages 109-116, June 1988.
The fundamental structure of RAID is the array. An array is a collection of drives that is configured, formatted and managed in a particular way. The number of drives in the array, and the way that data is split between them, is what determines the RAID level, the capacity of the array, and its overall performance and data protection characteristics.
An array appears to the operating system to be a single logical hard disk. RAID employs the technique of striping, which involves partitioning each drive's storage space into units ranging from a sector (512 bytes) up to several megabytes. The stripes of all the disks are interleaved and addressed in order.
In a single-user system where large records, such as medical or other scientific images are stored, the stripes are typically set up to be relatively small (perhaps 64 k bytes) so that a single record often spans all disks and can be accessed quickly by reading all disks at the same time.
In a multi-user system, better performance requires establishing a stripe wide enough to hold the typical or maximum size record. This allows overlapped disk I/O (Input/Output) across drives.
Most modern, mid-range to high-end disk storage systems are arranged as RAID configurations. A number of RAID levels are known. RAID-0 “stripes” data across the disks. RAID-1 includes sets of N data disks and N mirror disks for storing copies of the data disks. RAID-3 includes sets of N data disks and one parity disk, and is accessed with synchronized spindles with hardware used to do the striping on the fly. RAID-4 also includes sets of N+1 disks, however, data transfers are performed in multi-block operations. RAID-5 distributes parity data across all disks in each set of N+1 disks. RAID levels 10, 30, 40, and 50 are hybrid levels that combine features of level 0, with features of levels 1, 3, and 5. One description of RAID types can be found at
http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci214332,00.html.
In the early days of RAID, fault tolerance was provided through redundancy. However, problems occurred in situations where a drive failed in a system that runs 24 hours a day, 7 days a week or in a system that runs 12 hours a day but had a drive go bad first thing in the morning. The redundancy would let the array continue to function, but in a degraded state. The hard disks were typically installed deep inside the server case. This required the case to be opened to access the failed drive and replace it. In order to change out the failed drive, the other drives in the array would have to be powered off, interrupting all users of the system.
If a drive fails in a RAID array that includes redundancy, it is desirable to replace the drive immediately so the array can be returned to normal operation. There are two reasons for this: fault tolerance and performance. If the drive is running in a degraded mode due to a drive failure, until the drive is replaced, most RAID levels will be running with no fault protection at all. At the same time, the performance of the array will most likely be reduced, sometimes substantially.
An important feature that allows availability to remain high when hardware fails and must be replaced is drive swapping. Strictly speaking, the term “drive swapping” simply refers to changing one drive for another. There are several types of drive swapping available.
“Hot Swap”: A true hot swap is defined as one where the failed drive can be replaced while the rest of the system remains completely uninterrupted. This means the system carries on functioning, the bus keeps transferring data, and the hardware change is completely transparent.
“Warm Swap”: In a so-called warm swap, the power remains on to the hardware and the operating system continues to function, but all activity must be stopped on the bus to which the device is connected.
“Cold Swap”: With a cold swap, the system must be powered off before swapping out the disk drive.
Another approach to dealing with a bad drive is through the use of “hot spares.” One or more additional drives are attached to the controller but are not used by I/O operations to the array. If a failure occurs, the controller can use the spare drive as a replacement for the bad drive.
The main advantage that hot sparing has over hot swapping is that with a controller that supports hot sparing, the rebuild will be automatic. The controller detects that a drive has failed, disables the failed drive, and immediately rebuilds the data onto the hot spare. This is an advantage for anyone managing many arrays, or for systems that run unattended.
Hot sparing and hot swapping are independent but not mutually exclusive. They will work together, and often are used in that way. However, sparing is particularly important if the system does not have hot swap (or warm swap) capability. The reason is that sparing will allow the array to get back into normal operating mode quickly, reducing the time that the array must operate while it is vulnerable to a disk failure. At any time either during rebuild to the hot spare or after rebuild, the failed drive can be swapped with a new drive. Following the replacement, the new drive is usually assigned to the original array as a new-hot spare.
When a RAID array disk drive goes bad, the system must make changes to the configuration settings to prevent further writes and reads to and from the bad drive. Whenever a configuration change happens, the configuration changes have to be written out to all of the disks in the RAID array.
When the operating system or an application wants to access data on a hard disk before it has loaded native drivers for disk access, it traditionally employs BIOS services to do this. BIOS is the abbreviation for Basic Input/Output System. Various vendors, such as Acer America, San Jose, Calif., American Megatrends Inc., Norcross, Ga., and Phoenix Technologies Ltd., Milpatis, Calif., among many others, have their own versions of BIOS. The BIOS provides basic input and output routines for communicating between the software and the peripherals such as the keyboard, screen and the disk drive.
The BIOS is built-in software that determines what a computer can do without accessing programs from a disk. The BIOS generally contains all the code required to control the keyboard, display screen, disk drives, serial communications, and a number of miscellaneous functions. While the access is not necessarily optimal, it is done through an easy to use interface. Minimal code can access these devices until the more optimal drivers take over.
The BIOS is typically placed on a ROM (Read Only Memory) chip that comes with the computer (it is often called a ROM BIOS). This ensures that the BIOS will always be available and will not be damaged by disk failures. It also makes it possible for a computer to boot itself.
When a drive failure occurs, it is necessary to make a configuration change to the array. If this is not done, applications will continue to write to and read from the bad drive. This will inevitably result in data corruption. However, there is only a limited amount of space in system memory, which makes it extremely difficult to manage configuration changes during boot up.
Every RAID controller uses configuration data to store the array information. This is known as controller metadata. The configuration information includes, among other things, the RAID level, how many disks in the array, the drive name or number, the location of the data, especially the starting location, and any other data required to enable the RAID controller to configure the RAID sets and provide the correct data back to the user.
Configuration data is modified on a regular basis. This is especially, but not only, the case where the system contains multiple RAID controllers and multiple arrays. It is not uncommon for RAID controllers to change the number of disks in their array. For example, the controller may add disks to its array to change from a RAID-1 array to a RAID-5 array. In another example, a disk in a given array may develop a fault and must be taken out of service. In either situation, the configuration data for the RAID array changes. The stored configuration data must be updated. Configuration data is normally stored on disk. Typically, the only configuration data stored is the most recently applied configuration.
When users perform complex tasks, they sometimes make mistakes that result in missing RAID arrays or lost data. It is very difficult to find out what happened and recover the missing arrays and data. This can be devastating to a business that has large numbers of records stored in the arrays. It is imperative that there be some way to recover the missing or lost data. Therefore, what is needed is a method and system to easily reconfigure RAID arrays and to recover missing arrays and data.